All types of fiber are subject to polarization dispersion: a pulse sent by a sending terminal is deformed when it is received. Its duration is greater than its original duration. The deformation is due to the fact that the optical signal is depolarized during transmission. The signal received at the end of the connecting fiber may be considered to comprise two orthogonal components, one corresponding to a maximum propagation speed polarization state (fastest main polarization state) and the other corresponding to a minimum propagation speed polarization state (slowest main polarization state).
In other words, a pulse signal received at the end of the connecting fiber may be considered to comprise a first pulse signal polarized in accordance with an advanced polarization state and arriving first and a second pulse signal propagating in accordance with a retarded propagation state and arriving with an instantaneous differential delay that depends in particular on the length of the connecting fiber.
If the sending terminal sends an optical signal consisting of a very short pulse, the optical signal received by the receiving terminal comprises two successive pulses polarized orthogonally and having a relative time shift equal to the instantaneous differential delay. This delay can be 20 ps for a 100 km link comprising a monomode fiber of the kind manufactured a few years ago.
Deformation of the pulses received by the receiving terminal can cause errors in decoding the transmitted data and polarization dispersion is therefore a factor limiting the performance of optical links, whether analog or digital.
We now know how to fabricate monomode fibers with low polarization dispersion (approximately 0.05 ps/km.sup.1/2). However, a high proportion of monomode fibers installed in the last decade have very high polarization dispersion, which constitutes a major technical obstacle to propagation of the transmitted bit rates. Furthermore, if the bit rate race continues, there is nothing to prevent this problem appearing in the low polarization dispersion fibers that we now know how to produce.
We know how to make fibers with high polarization dispersion, also known as maintained polarization fibers, which enable a fixed differential delay to be achieved through the use of short segments. By judiciously placing a component of this kind (or any arrangement generating a differential delay between two orthogonal polarization modes) in series with a transmission link subject to high polarization dispersion, it is possible to compensate the polarization dispersion. This can be achieved either using a maintained polarization fiber having the same differential delay as the link, but interchanging the slow and fast main polarization states, or by having a main polarization state of the combination of the link and the maintained polarization fiber coincide with the polarization state of the sending source. To achieve this a polarization controller is placed between the link and the maintained polarization fiber.
The value of the differential delay and the main polarization states of a link vary in time in accordance with many factors, such as vibration and temperature. A compensator system must therefore necessarily be adaptive and the differential delay of the maintained polarization fiber must be made at least equal to all differential delay values to be compensated.
Polarization dispersion is a difficult problem to solve in the context of upgrading existing optical fiber networks with channels having bit rates of 10 Gbit/s and above.
It is estimated that the maximum polarization dispersion that can be tolerated is 10% of the duration of a bit, for example 10 ps at 10 Gbit/s and only 2.5 ps at 40 Gbit/s.
Polarization dispersion compensator systems have been designed. However, there is no means of solving the problem of polarization dispersion in a wavelength-division multiplex network other than juxtaposing identical single-channel compensator systems after demultiplexing. One example of this is the system disclosed in French Patent Application No. 96 16194 filed Dec. 30, 1996 by the Applicant and concerning a single-channel transmission system of this kind.